Metallic porous bodies having a three-dimensional net-like structure are used in the fields of various articles, such as various filters, catalyst carriers, and electrodes for batteries. For example, CELMET ((registered trade mark) manufactured by Sumitomo Electric Industries, Ltd.), which is made of a three-dimensional net-like nickel porous body (hereinafter referred to as a “nickel porous body”), is used as a material for an electrode of a battery such as a nickel hydrogen battery or a nickel cadmium battery. The CELMET is a metallic porous body having continuous pores, and is characterized by having a higher porosity (90% or more) than other porous bodies such as a metallic nonwoven fabric. This is obtained by forming a nickel layer onto the surface of a skeleton of a resin porous body having continuous pores, such as a urethane foam, treating the workpiece thermally to decompose the resin foam shaped body, and further reducing the nickel. The formation of the nickel layer is attained by painting a carbon powder or some other onto the skeleton surface of the resin foam shaped body to subject the surface to an electrically conduction treatment, and then electroplating the workpiece to precipitate nickel.
In the meantime, aluminum has excellent characteristics, such as electroconductivity, corrosion resistance and lightness, similarly to nickel. About the use thereof for batteries, the following is used as a positive electrode of a lithium battery: a member in which an active material such as lithium cobaltate is painted on surfaces of an aluminum foil. In order to improve the capacity of the positive electrode, it is conceived that a three-dimensional net-like aluminum porous body (hereinafter referred to as an “aluminum porous body”), wherein the surface area of the aluminum is made large, is used, and an active material is filled also into the aluminum. According to this form, the active material can be used even when the electrode is made thick, so that the electrode is improved in availability ratio of the active material per unit area.
As a method for producing an aluminum porous body, Patent Literature 1 describes a method of subjecting a three-dimensional net-like plastic base having internal spaces connected to each other to an aluminum vapor deposition treatment by an arc ion plating method, thereby forming a metallic aluminum layer of 2 to 20 μm thickness.
It is stated that according to this method, an aluminum porous body of 2 to 20 μm thickness is obtained; however, the porous body is not easily produced so as to have a large area since the method is based on a vapor deposition method. Depending on the thickness or the porosity of the base, the layer is not easily formed so as to be even inside the porous body. Moreover, the forming velocity of the aluminum layer is small; costs for the production increase because of a high price of facilities, and other causes; and other problems remain. Furthermore, when aluminum is made into a thick film, it is feared that the film is cracked or aluminum peels off.
Patent Literature 2 describes a method of yielding an aluminum porous body by forming a film made of a metal (such as copper) capable of producing a eutectic alloy at the melting point of aluminum, or lower onto a skeleton of a resin foam shaped body having a three-dimensional net-like structure, painting an aluminum paste thereon, and treating the workpiece thermally at a temperature of 550° C. or higher and 750° C. or lower in a non-oxidizing atmosphere, thereby removing the organic component (the resin foam) and sintering the aluminum powder.
However, according to this method, a layer that is combined with aluminum to form a eutectic alloy is unfavorably formed, so that an aluminum layer high in purity cannot be formed.
In a different method, it is conceived that a resin foam shaped body having a three-dimensional net-like structure is plated with aluminum. The method of electroplating with aluminum is itself known. However, in plating with aluminum, the affinity of aluminum with oxygen is large, and the potential thereof is lower than that of hydrogen; thus, it is difficult that electroplating therewith is conducted in a plating bath of an aqueous solution type. For this reason, about electroplating with aluminum, nonaqueous solution baths for plating have been hitherto investigated. For example, as a technique for plating a metal surface with aluminum in order to prevent the surface from being oxidized, Patent Literature 3 discloses an aluminum electroplating method of using a low-melting-point composition wherein an onium halide and an aluminum halide are mixed with each other and molten, as a plating bath, to precipitate aluminum onto a negative electrode while the water content in the bath is kept into 2% by weight or less.
However, about electroplating with aluminum, only a metal surface can be plated. There has not been known a method of electroplating a resin shaped body surface therewith, in particular, a method of electroplating a surface of a resin porous body having a three-dimensional net-like structure therewith.